Electric latent images formed by an electrophotographic apparatus, electrostatic recording apparatus or the like have heretofore been first developed with a toner. After the toner image thus formed is then transferred to a transfer medium such as paper as needed, the unfixed toner image is fixed by any of various methods such as heating, pressing and use of solvent vapor.
In general, a toner has heretofore been produced by melting and mixing a colorant, a charge control agent, an offset preventing agent and the like in a thermoplastic resin, uniformly dispersing them in the thermoplastic resin to prepare a composition, grinding the composition and then classifying the ground product. According to this production process (i.e., grinding process), a toner having a measure of excellent properties can be produced. However, there is a limit to the selection of materials for the toner. For example, the composition formed by the above melting and mixing must be such that can be ground and classified by economically usable apparatus. This requirement obliges the composition obtained by the melting and mixing to become fully brittle. For this reason, the toner obtained by such a grinding process involves the following disadvantage. When the above composition is ground into particles, the particles formed tend to have a wide particle diameter distribution. When it is intended to obtain a copy image having good resolution and gradation, therefore, fine powder having a particle diameter of 5 .mu.m or smaller and coarse powder having a particle diameter of 20 .mu.m or greater must be removed from the ground product by classification, so that the yield of the toner becomes very low.
Besides, in the grinding process, it is difficult to uniformly disperse solid fine particles such as the colorant, charge control agent and offset preventing agent in the thermoplastic resin. The unevenly dispersed state of the solid fine particles may form the cause that fog is increased, or image density is lowered. The uneven dispersion of these solid fine particles in the grinding process adversely affects the flowability, triboelectrification properties and the like of the resulting toner to a great extent and influences properties of the toner, such as developing characteristics and durability. Accordingly, in the grinding process, it is necessary to take good care to uniformly disperse these solid fine particles.
In order to overcome these problems in the grinding process, processes of producing a toner by suspension polymerization have been proposed in recent years. In the suspension polymerization process, a monomer composition with a polymerizable monomer, a colorant, a charge control agent, an offset preventing agent, a polymerization initiator and the like dissolved or dispersed uniformly is poured into water or an aqueous dispersion medium composed principally of water, which contains a dispersion stabilizer, and dispersed therein by means of a mixing device capable of mixing with high shearing force to form fine droplets of the polymerizable monomer composition, and the droplets are then polymerized, thereby forming toner particles (i.e., polymerized toner).
According to the suspension polymerization process, the colorant, charge control agent, offset preventing agent and the like are added to the monomer, which is a low-viscosity liquid, to disperse them therein. Therefore, a satisfactory dispersing quality compared with the grinding process, in which such components are dispersed in the resin, can be ensured. According to the suspension polymerization process, toner particles having the desired particle diameters can be generally obtained at a yield of at least 90%, and so such a process has an economical advantage over the grinding process. As described above, the problems involved in the grinding process have been able to be solved by the use of the suspension polymerization process, so that a toner, which can provide images excellent in image properties such as resolution and fog due to the extremely sharp particle diameter distribution of the polymer particles and good electric properties thereof, has been able to be produced economically.
In recent years, it has been attempted to permit reduction of demand power in copying machines, printers and the like of an electrophotographic system making use of a toner. Among the steps of the electrophotographic system, a step, in which energy is particularly demanded, is the so-called fixing step of fixing a toner after transferring the toner from a photosensitive member to a transfer medium such as paper. In the fixing step, a heating roll heated to at least 150.degree. C. is generally used, and electric power is used as an energy source therefor. There is a demand for lowering the temperature of the heating roll from the viewpoint of energy saving. In order to lower the temperature of the heating roll, it is necessary to lower the fixing temperature of the toner itself.
In order to meet requirements from the copying machines and the like in the design of a toner, it is only necessary to lower a glass transition temperature of the toner. When the glass transition temperature of the toner is lowed, however, the toner becomes poor in the so-called shelf stability in that particles themselves of the toner tend to undergo blocking during storage, or in a toner box, to aggregate.
In the case of color toners used in the electrophotographic system on the other hand, color images have come to be often used in OHP sheets for presentations in various meetings or conferences in recent years. Therefore, the color toners have been required to have excellent permeability through OHP. In order to meet the excellent permeability through OHP, it is necessary for the toners to uniformly melt on a OHP sheet. Therefore, the melt viscosity of each toner at about the fixing temperature thereof must be designed low compared with the conventional toners. Means for lowering the melt viscosity of the toner include a method in which the molecular weight or glass transition temperature of a binder resin used is lowered compared with the binder resins for the conventional toners. In any of these methods, however, the toner becomes poor in shelf stability because the toner tends to undergo blocking.
As described above, there is an adverse correlation between the means for lowering the fixing temperature of a toner, or improving the permeability through OHP and the shelf stability of the toner. As a means for solving this adverse correlation, there has heretofore been proposed the so-called capsule type toner in which toner particles are coated with a polymer having a high glass transition temperature, thereby solving the problem of shelf stability.
As a production process of the capsule type toner, for example, Japanese Patent Application Laid-Open No. 173552/1985 has proposed a process in which a coating layer composed of a colorant, magnetic particles or a conductive agent and a binder resin is formed on the surfaces of core particles by means of a Jet mill. When core particles having a low glass transition temperature are used in this method, however, the core particles themselves tend to undergo aggregation. Accordingly, this method cannot be applied to such core particles.
Japanese Patent Application Laid-Open No. 259657/1990 has proposed a process for producing a toner for electrophotography, in which crosslinked toner particles prepared by suspension polymerization are added to a solution with an encapsulating polymer, a charge control agent and a parting agent dissolved in an organic solvent, and a poor solvent is then added to the resultant mixture to form a coating film of the encapsulating polymer containing the charge control agent and the parting agent on surfaces of the crosslinked toner particles. This process however involves a problem that since the solubility of the encapsulating polymer is reduced by the addition of the poor solvent to deposit it on the surfaces of the crosslinked toner particles, the capsule wall formed on the surface of the crosslinked toner particle becomes uneven in thickness.
Japanese Patent Application Laid-Open No. 45558/1982 has proposed a process for producing a toner for development of electrostatic latent images, in which core particles formed by polymerization are mixed and dispersed in a 1-40 wt. % aqueous latex solution, and a water-soluble inorganic salt is then added to the dispersion to form a coating layer composed of fine particles obtained by emulsion polymerization on surfaces of the core particles. However, this process has involved a drawback that the environmental dependence of charge properties of the resultant toner becomes great due to the influence of the surfactant and inorganic salt remaining on the fine particles, and in particular, the charge properties are deteriorated under high-temperature and high-humidity conditions.